High dynamic range (HDR) file is a special image file format and each pixel comprises the information of RGB and the real brightness of the point. Generally, the grey scale of each pixel of the image file ranging only from 0 to 255 is not enough because the difference between the brightness scale/grey scale of the sun and a black object is much more than 256. The cloud and the sun in a common daytime scene image might be presented in the same grey scale/brightness scale (as pure white), but in fact there is a huge difference between the brightness scale of the cloud and the sun. Thus, the common image file format doesn't precisely record the true situation in the real world.
High dynamic range refers to the relative ratio between the lightest and darkest area of a scene. The brightness of the actual scene recorded by the high dynamic range image is over than 256 levels but the area which exceeds can't be seen on the display. For example, while opening an image with the scene from the indoor to the outside of the window in Photoshop, the outside area of the window shows a plenty of white color without many details due to the overexposure. Dimming the light won't show more details but only turn the white color into the grey color. If the same scene is recorded as the high dynamic range image, reducing the exposure will be able to presents more details in the area previously shown as the white color.
The traditional method of high dynamic range image synthesis usually uses the fixed exposure values to shoot several images of the same scene. Images in different exposure conditions are combined to integrate the image dynamic status of the scene, and the difference of the exposure value of each image ranges from 1 to 2 exposure compensations or fewer. Because the dynamics of each image is limited, such traditional method requires more images to synthesize the high dynamic range image. A common camera can not process more than three images in the short period due to the limited processing capability. Therefore, the present technology selects two or three images with fixed exposure compensation to synthesize the high dynamic range image. However, an expected high dynamic range image can't always be retrieved because the appropriate exposure compensation value of the scene can not be set for image shooting or image processing. Thus, developing an ideal image capturing device and an image synthesis method to synthesize a high dynamic range image is an emerging issue in the market.